SPAWNING AND SETTING OF OLYMPIA OYSTERS 
497 
hags of shells as previously described. Range of tide is also given. During this j r car 
most of the spawning took place from the middle of May until the middle of June, 
and during July there was some further active spawning. In some respects the record 
of setting resembles that of spawning, though the break between the two setting 
periods may not readily be correlated with a comparable cessation of spawning. If 
the second setting period is traceable to larvae resulting from July spawning the 
mortality of larvae produced during the first spawning period was tremendously 
larger, for the later period of setting was very intense. 
In the graph referring to the year 1934 (fig. 41) the picture is somewhat similar, 
though one can hardly consider that there was a second distinct period of spawning. 
In this figure the correlation between tidal periods and setting is strikingly shown, 
while in figure 40 the second setting period appears to be correlated with the neap 
tides. However, in 
the latter case it is 
not quite correct to 
plot tides in this 
manner, for at the 
time of the second 
major set there 
were very low tides, 
but the high tides 
were not great, so 
that the total range 
shown is small. In 
both years, which 
were selected for 
presentation be- 
cause they represent marked differences in time of spawning and setting, the seasons 
of reproduction cover about 5 months. 
It may be noted that setting begins during the third period of tides following the 
beginning of spawning, and also that for 5 seasons, the second major setting period 
takes place during the third and fourth spring tide periods following that when the 
first set occurs. The time intervals seem to be predetermined, either by the spawning 
activity or by cyclic changes in the water which are correlated with tidal periodicity. 
DISCUSSION 
In the foregoing account various phases of the spawning and setting activities of 
the Olympia oyster have been described with particular reference to their application 
to commercial cultivation. Larvae of this viviparous species develop slowly within 
the maternal brood chamber, or that portion of the mantle chamber which contains 
the palps and the anterior ends of the gills, and require an average time of about 10 
days before they reach the size at which they are normally discharged into the open 
water. While eggs spawned by the female of 0. virginica and other viviparous species 
develop to the trochophore, or earliest swimming stage, within a few hours, in the 
case of 0. lurida the same stage is not reached for about 4 days. Rapid early develop- 
ment is characteristic of those species which discharge the eggs directly into the open 
water, in contrast with the viviparous Olympia oyster which protects the embryos. 
Oyster culture in Puget Sound is somewhat different from that in other parts of 
the United States in that the range of tides is greater, with a maximum range of 20 
feet. The oyster grounds are above the extreme low-tide level and are surrounded 
Figure 41.— Frequency of spawning and setting during season of 1934 in Oyster Bay. Setting is 
indicated by a trend line derived from values given in figure 26. Tidal range is also shown. 
